The chronically critically ill are a vulnerable population of patients with persistent multi-organ failure; they represent the sickest-sick. This population, although small, is increasing in number; they present a significant burden to intensive care infrastructure and are expensive to care for. The current state of the medical literature for the chronically critically ill population is limited to small cohorts from individua tertiary care hospitals; national estimates for the epidemiology, outcomes and costs of this population are needed to inform policy making and resource allocation in the future. I propose to create a definition of chronic critical illness for use in administrative datasets. I will describethe population both in cross-section and over time using two nationally representative datasets. I will then identify the burden of pre-morbid disease, describe outcomes and estimate costs. I additionally propose to use a novel device to capture body composition at the time of intensive care unit admission to predict progression to chronic critical illness and ultimately provide bette prognostic information to patients and their caregivers to aid in resource allocation and determination of goals of care.